Immunosuppression and senescence in dung beetles exposed to ivermectin

Immunosuppression and premature senescence are main risks of exposure to toxic compounds that might define individual longevity and the fate of natural animal populations. However, these sublethal effects have not been studied in insects that are of fundamental importance for human economy and well-being such as dung beetles. We exposed adult dung beetles Euoniticellus intermedius to ivermectin, a commonly used antiparasitic drug in cattle that is excreted in dung, and measured immune activity through phenoloxidase (PO) and its zymogen, prophenoloxidase (proPO), in males and females of different ages. We predicted that both ivermectin exposure and age would reduce immune activity, with the negative effect of ivermectin being more pronounced in older individuals, revealing immunosenescence. Despite PO and proPO activities decreased with age and ivermectin exposure, ivermectin effects on these immune mechanisms were mainly constant across ages, revealing that immunosenescence is not an effect of ivermectin exposure. Whereas males suffered a reduction in PO and proPO with ivermectin and age, female proPO was not affected by ivermectin or age. This potentially reveals a strategy of self-care adopted by females to prioritize immune function when facing stressful conditions. Among sublethal effects caused by ivermectin in dung beetles, immunosuppression might be a main physiological driver of population declines in non-target beneficial fauna from contaminated environments.